This invention relates to a hydraulic shock absorber and more particularly to an improved, variable rate, hydraulic shock absorber.
There is a very popular type of hydraulic shock absorber wherein damping forces are obtained by positioning a piston in a sliding cylinder and to divide the cylinder into a pair of oppositely disposed fluid chambers. The piston is provided with two series of apertures each of which permits flow from one chamber to the other. By employing a plurality of stacked type plate valves on the opposite sides of the piston, the flow through the respective apertures can be controlled.
This type of device is quite effective, but as shown in the dotted line view of FIG. 1 has a generally linear damping force characteristic with respect to displacement or piston speed. That is, as the speed of the piston increases, the damping force increases linearly as shown by the curve D. Although such devices are effective, they tend to require large suspension travels in order to achieve the desired ride characteristics.
There has been proposed, therefore, a type of damping arrangement as shown in FIG. 2. This type of damping arrangement gives a speed to damping force characteristic as shown by the solid line curve C in FIG. 1. During the original portion of the suspension travel, there is a relatively high damping force as indicated by the line C.sub.1. At a particular point P, which occurs at the piston speed Vo, the damping characteristic becomes less progressive in nature as seen by the curve C.sub.2. This provides very effective damping under small loads and still permits a soft ride without necessitating large suspension travel.
The type of construction employed to achieve the damping curve C in FIG. 1 is obtained by the structure as shown in FIG. 2. As seen in this figure, there is a piston rod 11 which is connected to one of the suspended elements in a manner which will be described in more detail later when the preferred embodiment is described. A piston 12 is connected in a suitable manner to the piston rod and is provided with a first series of apertures 13 that are spaced so that their axes lie on a circle set at a fixed distance from the axis of the piston rod 12. The openings 13 permit flow from the chamber which exists above the piston 12 to a chamber 14 that is formed below it by an associated cylinder.
The flow through the apertures 13 is controlled by a stack plate type valve, indicated generally by the reference numeral 15 and which has a plurality of plate type valve elements 15a, 15b, 15c, 15d and 15e. These valve elements 15a through 15e are held on the piston rod by a retainer ring 16 in a manner known in the art. The structure as thus far described is like the conventional structure and would provide a damping force as shown by the curve D in FIG. 1.
In order to provide the knee-type damping curve C of FIG. 1, a pre-load shim 17 is interposed between the plate type valve elements 15a and 15b. This shim 17 is selected of a desired thickness so as to deflect the valve plates 15b, 15c, 15d and 15e as shown in FIG. 2. This, in effect, gives a pre-load that biases the valve plate 15a in its closed position.
A small bypass passage 18 is formed in the piston 12 and communicates the passages 13 with the chamber 14 bypassing the valve 15. Thus, during initial upward movement of the piston 12 relative to the associated cylinder, the valve 15 will maintain in a closed position and the size of the orifice 18 will determine the shape of the damping curve C.sub.1. However, at the point P when the piston velocity reaches the velocity Vo, sufficient pressure will be generated so as to overcome the pre-load on the valve element 15a and it will open and the damping curve C.sub.2 will then result.
As may be seen, the shim 17 in the conventional structure is disposed so that it is radially outwardly of the series of apertures 13 and hence the pressure or piston velocity at which the valve element 15 will open is fixed within a limited range. This is not desirable because it would be preferred to be able to obtain a damping curve that would have more adjustability so as to suit varying conditions.
It is, therefore, a principal object of this invention to provide an improved variable rate shock absorber of this type.
It is a further object of this invention to provide a shock absorber having a rate that may be varied in a greater range than the prior art type of constructions.